Because of the characteristics of hydraulic pressure generating systems, the pressure of hydraulic fluid for driving hydraulic devices involves pulsations, which therefore entail the problem of failing to assure smooth operation of the hydraulic device.
For example, when hydraulic pressure is produced by a cylinder apparatus, a high hydraulic pressure is generated in the oil feed process in which the plunger advances in the hydraulic chamber to force out the hydraulic fluid, whereas the generation of hydraulic pressure ceases in the suction process in which the plunger starts to retract from the end of its forward stroke to draw in the fluid from an oil supply source. thus resulting in a pulsation.
The pulsations of hydraulic pressure can be eliminated by the use of an accumulator for temporarily storing the pressure delivered from the hydraulic pressure generating apparatus, but this entails the drawback that the apparatus becomes expensive and large-sized.
At pharmaceutical plants and factories handling flammable liquids, there is a special need for hydraulic pressure generating apparatus which use no electric motor because electric motors are likely to cause ignition. To meet this need, the present inventor has already proposed an apparatus wherein a hydraulic cylinder unit is connected directly to a pneumatic cylinder unit, and a plunger projecting from the piston of the pneumatic cylinder unit is slidably intimately fitted in the hydraulic chamber of the hydraulic cylinder unit so that the piston, when reciprocated, drives the plunger to generate hydraulic pressure (Published Unexamined Japanese Patent Application SHO No. 53-43210).
The apparatus includes two pistons which are made slightly different in the timing of reciprocation and of the resulting generation of hydraulic pressure to produce a combined hydraulic pressure with reduced pulsations. Although thus excellent, the apparatus still permits slight pulsations to remain in the hydraulic pressure.